An impregnated cathode has a basic structure in which pores of a sintered body of porous metal (pellet) are impregnated with an electron emitting material. A method for manufacturing an impregnated cathode comprises the steps of: press molding powder of a high melting point metal such as tungsten, etc.; then sintering the press molded product to form a reducing substrate having a proper porosity; and then impregnating the pores of the substrate with molten electron emitting material comprising BaO, CaO and Al.sub.2 O.sub.3 as the main components. Thus, a cathode pellet is obtained. This cathode pellet is impregnated with emitting material in an amount corresponding to the volume of the sintered body and the porosity, i.e. the volume of pores.
The principle of operation of the cathode pellet will be explained below. When the cathode pellet is subjected to a high temperature activation, BaO is reduced by the pellet to generate free Ba. This free Ba thermally diffuses in pores and reaches the surface of the pellet. Then, the free Ba thermally diffuses on the surface of the pellet, to thus form a Ba monoatomic layer on the surface of the pellet. At this time, a monoatomic layer spreads to cover an area corresponding to the difference between an amount of Ba evaporated from the monolayer, which is dependent upon the temperature of the pellet, and an amount of Ba supplied from the inside of the pellet. This Ba monoatomic layer reduces the effective work function that is involved in an electron emission from 4 to 5 eV of the metal itself constituting the pellet to about 2 eV. Consequently, excellent thermionic emission is provided.
If little Ba is supplied from the inside of the pellet at the time of the operation, a necessary and sufficient area of Ba monoatomic layer cannot be formed, causing a deficiency of emission. Moreover, there arise some problems, for example, the activation takes a long time, etc.
On the contrary, if too much Ba is supplied, Ba evaporated from the surface of the pellet is increased, so that the BaO impregnated in the pellet is consumed in a short time and in turn the lifetime is shortened. Furthermore, the evaporated Ba is deposited on a counter electrode, causing unnecessary electron emission, etc.
The most important point of the operation of the impregnated cathode is to form a necessary and sufficient Ba monoatomic layer in an early stage and to keep it for a long time. The factors for forming a Ba monoatomic layer include: the amount of impregnated BaO; the reducing rate of the impregnated BaO being reduced by the pellet; the thermal diffusion velocity of free Ba in pores; and the surface thermal diffusion rate of Ba on an electron emitting face.
The design parameters for controlling the operations are: the amount of impregnation of electron emitting material; the porosity of the pellet and the spatial distribution of pores; and the cleanness of the electron emitting face, more specifically, an absence of extra electron emitting material attached to the electron emitting face. The most important thing for mass production is to control these parameters with high precision and with less variation.
Based on the above mentioned background of the principle, Publication of Japanese Patent Application (Tokko Sho) No. 44-10810 discloses an impregnated cathode, in which the evaporation of extra electron emitting material can be inhibited, the leak of current in an insulating portion of an electron gun can be reduced, and an excellent state of Ba monoatomic layer can be maintained for a long time and in turn its lifetime can be extended.
The above mentioned structure is a two-layer structure comprising a first layer having a low porosity on the side of the electron emitting face of the pellet, wherein the evaporation of the electron emitting material is inhibited; and a second layer having a high porosity formed below the first layer. According to such a two-layer structure, even after the Ba supply capacity of the first layer is exhausted (i.e. after the lifetime), Ba can be supplied from the second layer to the first layer. Consequently, the lifetime of the pellet is further extended as compared with the lifetime the first layer has naturally.
Furthermore, Publication of Japanese Patent Application (Tokkai Hei) No. 6-103885 suggests that the surface roughness of the substrate be not more than 5 .mu.m, more preferably that the substrate be perfectly smooth, so as to easily remove the attached extra electron emitting material after impregnation.
Furthermore, Publication of Japanese Patent Application (Tokkai Sho) No. 58-87735 discloses a manufacturing method in which compressed electron emitting materials placed on the upper surfaces of the individual pellets are melted and impregnated in order to ensure the amount of impregnation of the electron emitting material.
Furthermore, Publication of Japanese Patent Application (Tokkai Hei) No. 6-103885 discloses a method of mass production in which the amount of the impregnated electron emitting materials is kept stable by classifying metal raw material powder of the pellet and controlling the porosity of the pellet.
Furthermore, a mechanical method using a brush, a metal-clad needle, etc., a polishing method by means of cutting, etc., and ultrasonic cleaning in water, etc. have been conventionally suggested.
Furthermore, Publication of Japanese Patent Application (Tokkai Sho) No. 50-103967 discloses a method in which a pellet is provided on the specific jigs one by one and then washed by ultrasonic cleaning in clean water.
However, the above mentioned conventional impregnated cathodes have the following problems.
(1) In order to manufacture the impregnated cathode having a two-layer structure, it is necessary to use two different kinds of raw material powders or to carry out press molding twice. Consequently, the production process is complicated. PA1 (2) In the method in which a pellet is treated one by one or the raw material powder is classified, the productivity is poor and mass production is difficult. PA1 (3) The method of mechanically removing extra electron emitting materials by using a brush, metallic needle, etc., is difficult to carry out. Furthermore, a treatment is necessary for each pellet, so that mass production is difficult. PA1 (4) The manufacturing process in which the sintered pellets are provided on the specific jig one by one is complicated. It takes not less than 1 hour to perfectly remove extra electron emitting materials by way of only the ultrasonic cleaning method. Consequently mass production is difficult.